A green turtle off Borneo in Blue Planet II - WARNING: Use of this copyright image is subject to the terms of use of BBC Pictures' Digital Picture

Nightmare fuel was served up in great, writhing dollops as David Attenborough’s Blue Planet II (BBC One) plunged into the balmy waters of the world’s coral reefs.

A metre-long worm with dagger-like teeth rose spectrally from the seabed to bag a passing snack. A cuttlefish hypnotised its prey by turning itself into a spooky underwater LED display. Dead-eyed sharks churned around one another as they gorged on spawning Grouper fish – a real life Sharknado that, for all the cinematic verve, soared to five-star creepiness.

Ratcheting up the chills was David Attenborough’s silky voiceover – his narration humming along with unnerving calmness as the fish-on-fish violence intensified. It was a reminder that, though praised for their groundbreaking dazzle, Attenborough’s BBC documentaries have never flinched from the tooth and claw – or tooth and tentacle– grisliness of life in the wild.

A meter-long bobbit wormCredit: BBC

And that was just a build up to the bittersweet coda, in which the camera glided over acres of ghostly bleached coral while Attenborough warned that global warming threatened an entire ecosystem with extinction. The Bobbit worm – of the hook-like fangs and insatiable carnivorous instinct – might have been the stuff of bad dreams but the true monster, it was implied, was humanity and its relentless compulsion to wreak environmental havoc.

Alongside the horror-show stuff care was taken to warm our cockles too. In Australia’s Great Barrier Reef an octopus and a predatory fish teamed up, buddy movie-style, to flush out tasty nibbles. Later, we were introduced to a petite turtle treating herself to top level underwater pampering courtesy of fish which feasted on the parasites that had taken up residency on her exterior.

Blue Planet II: Highlights of the new BBC natural history series, in pictures

As is often the case with Attenborough documentaries, the scene was both cute – look, a turtle going to the spa! – and also icky. How many parasites, you were invited to wonder, lived in those neck folds?

There were also several obligatory blockbuster moments showcasing the virtuosity of the BBC Natural History Unit: an overhead drone shot of Manta Ray twirling in an elegant feeding vortex off the Maldives, those gunmetal reef sharks descending on frantically reproducing groupers in the South Pacific

These images were gorgeous but viscerally eerie too – evidence that nature can be at once hauntingly beautiful and brutishly nasty. Nobody evokes that contradiction better than Attenborough. Here his sage intonations were the perfect accompaniment to an underwater odyssey that dazzled even as it occasionally made the blood run cold.